[c] semaphore implementation

The fundamental issue with your code is that you mix two APIs. Unfortunately online resources are not great at pointing this out, but there are two semaphore APIs on UNIX-like systems:

  • POSIX IPC API, which is a standard API
  • System V API, which is coming from the old Unix world, but practically available almost all Unix systems

Looking at the code above you used semget() from the System V API and tried to post through sem_post() which comes from the POSIX API. It is not possible to mix them.

To decide which semaphore API you want you don't have so many great resources. The simple best is the "Unix Network Programming" by Stevens. The section that you probably interested in is in Vol #2.

These two APIs are surprisingly different. Both support the textbook style semaphores but there are a few good and bad points in the System V API worth mentioning:

  • it builds on semaphore sets, so once you created an object with semget() that is a set of semaphores rather then a single one
  • the System V API allows you to do atomic operations on these sets. so you can modify or wait for multiple semaphores in a set
  • the SysV API allows you to wait for a semaphore to reach a threshold rather than only being non-zero. waiting for a non-zero threshold is also supported, but my previous sentence implies that
  • the semaphore resources are pretty limited on every unixes. you can check these with the 'ipcs' command
  • there is an undo feature of the System V semaphores, so you can make sure that abnormal program termination doesn't leave your semaphores in an undesired state